You can choose what eevee will evolve into with this simple cheat. It’s simple, all you need to do is name eevee after one of the original three eevee brothers from the original anime! Before you level your eevee you need to nickname eevee which ever eeveeloution you would like.
For Vaporeon you need to nickname your eevee rainer.
For Jolteon you need to nickname eevee Sparky.
For Flareon you need to nickname eevee Pyro.
Remember you need 25 eevee candy to evolve. You can get eevee candy by catching eevee, transferring eevee to professor. Or you can get quite a few eevee candy from hatching a eevee from a 10k egg. (Which yes it does suck if you do all that walking just for an eevee when you can catch eevee in the wild)